Compacting auxiliary agent for producing sinterable shaped parts from a metal powder

ABSTRACT

The invention relates to a process for producing sinterable metallic shaped parts from a metal powder, which is mixed with an auxiliary compacting agent containing at least in part components from the polyalkylene glycol family, is filled into a compacting mold and, following the compacting under pressure, is ejected from the mold as compacted shaped part.

DESCRIPTION

The problem encountered when producing metallic shaped parts with apowder-metallurgical process is that the shaped parts must be producedwith the highest possible density since the metallic powders areinitially filled into a mold cavity and are then compacted at highpressure with the aid of single-axle or multi-axle hydraulic ormechanical presses. A shaped part obtained in this way, which isgenerally referred to as a green compact, is subsequently sintered in athermal process, mostly in a protective atmosphere, so as to result in asolid, accurately dimensioned metal shaped part.

The density of the finished, sintered shaped part in this case dependsessentially on the green compact density that can be achieved. Incontrast to the compacting of ceramic powders, the metal powderparticles experience a plastic deformation, owing to their differentcrystalline structure and the number of movable lattice defectsconnected with this. With metallic powders, the sliding ability of theindividual particles relative to each other is reduced as a result ofthe particle geometry—also in contrast to ceramic powders—so that theloose bulk material in the mold already has a pore volume, which can beremoved completely only if extremely high forces are used for thecompacting operation. However, high compacting forces result in highwear of the compacting tool during the compacting operation and alsolead to increased sliding friction in the mold cavity during theejection of the completed green compact, so that higher ejection forceswith correspondingly increased wear must be generated in this case aswell. On the other hand, high ejection forces carry the danger of anundesirable local secondary compacting and the formation of cracks inthe green compact.

In order to avoid these disadvantages, a process was suggested in theEP-A-0 375 627, whereby a lubricant that is liquefied with a liquidsolvent is added to the metal powder to be compressed. The lubricantssuggested for this include metal stearates, particularly lithiumstearate or zinc stearate, as well as paraffin products, waxes, naturalor synthetic fat derivatives, which are first liquefied, e.g. withorganic paraffin solvents as liquid solvent. The disadvantage of thisprocess is that the dry metal powder must initially be mixed with atwo-component lubricant system, namely the stearates and the solvents,wherein this preliminary mixture for the most part must be homogeneous.Another disadvantage is that prior to filling the powder mixture intothe pressing mold, it must first be preheated to a relatively high heat,up to the range of the softening point for the lubricant used. Thisentails the danger of baking on while moving through the feeding devicesfor the mold. Following the completion of the compacting operation andthe ejection of the green compact, the lubricant must be vaporized in aseparate operation before the green compact can be heated to the actualsintering temperature. In the process, it cannot be avoided thatlubricant residues remain in the sintered body, which can also result indisadvantages, depending on the application and the type of pure oralloyed metal powder used.

An iron-based metallurgical powder composition is known from the EP 0559 987, which contains an organic binder for the iron-based powdercomponents and the alloy powder components. In order to improve thecompacting behavior, the organic binder contains a share of polyalkyleneoxide, which must have a molecular weight of at least 7000 g/mol.However, considerably higher molecular weights are preferred.

It is the object of the invention to improve the above-describedprocess.

This object is solved with a process for producing sinterable metallicshaped parts from a metal powder, mixed with an auxiliary compactingagent, which contains at least in part components from the polyalkyleneoxide family, is filled into a pressing mold and, following thecompacting under pressure, is ejected as compressed shaped part from thepressing mold. The use of auxiliary compacting agents containing atleast components from the polyalkylene oxide family, particularlypolyalkylene glycols and preferably polyethylene oxides, especially inthe form of polyethylene glycols, surprisingly showed that thecompacting forces required to achieve higher densities and higher greencompact strengths are much lower than for other auxiliary compactingagents. The forces needed for ejecting the compacted shaped part fromthe mold are also clearly reduced, so that the aforementioneddisadvantages of the known processes are avoided. Owing to the“lubrication” of the powder particles moving relative to each otherduring the compacting operation, the powder mixture does not require aspecial binder since it is possible to achieve a high green compactstrength during the compacting operation in addition to the highdensity, owing to a much higher “packing density” of the powderparticles and thus an increase in the direct contact between the metalparticles in the powder. A high green compact strength is alwaysdesirable if the green compact must be reworked further prior to thesintering. “Metal powder” within the meaning of the invention refers tothe powder mixture intended for the production of the shaped part,including all alloying agents and other admixtures, with the exceptionof the auxiliary compacting agent.

A special advantage of auxiliary compacting agents selected from thefamily of polyethylene oxides, particularly if these are used in theform of polyethylene glycols, is that the compacting parameters can beinfluenced through a corresponding selection of the molecular weight,that is to say with respect to the flow properties during the mixing andfilling of the mold, as well as with respect to the softening point andthus the temperature control and the material flow during the compactingoperation. It is particularly advantageous in this connection if thesoftening point for the auxiliary compacting agent suggested accordingto the invention is between 40° C. and 80° C., so that the temperatureadjusting at the tool during a continuous compacting operation for theseries production as a rule is sufficient to effect a trouble-free“flow” of the powder mixture during the filling of the mold as well asduring the compacting. Accordingly, the metal powder with addedauxiliary compacting agent can be filled into the mold at roomtemperature. Particularly for the series production, it may be useful ifthe compacting tool is heated accordingly to prevent possibleinterruptions in the series run. A controlled heating of the compactingtools to about 55° C. makes sense, so that the heating caused byfrictional heat as well as the cooling caused by interruptions in theoperation are taken into account and constant compacting conditions canbe specified. The handling of the metal powder is simplifiedconsiderably by this, particularly the filling operation because it ispossible to work with “cold” powder, meaning powder at room temperature.A baking on, lump formation and the like cannot occur since the metalpowder with mixed-in auxiliary compacting agent is heated only in themold. An additional preheating of the powder may be advisable forextremely large volume particles.

The low softening temperature additionally has the advantage thatimmediately after the filling operation, the shares of auxiliarycompacting agent in the metal powder, which make contact with the heatedmold walls, are initially warmed to the softening temperature. Thus,during the subsequent compacting operation, the relative movementsoccurring at the tool wall between powder filling and compacting toolare already “lubricated” and the friction in these regions is reduced.During the following operation where total compacting pressure isapplied, the complete powder filling is subsequently heated past thesoftening point as a result of the compacting pressure. Thus, even theinternal and relatively high relative movements in the metal powderfilling, which result from the particle geometry of the metal powder,are made easier by the effect of the auxiliary compacting agent withlubricating effect. Owing to the deformation of the powder particles andthe resulting increase in the packing density, a portion of theauxiliary compacting agent in the free-flowing state is additionallypushed toward the edge region, thereby resulting in a considerablereduction in the friction between the finished green compact and themold cavity wall during the ejection of the green compact. Thus, thesoftening temperature of the auxiliary compacting agent must be adjustedsuch that by taking into account the operating temperature during thecompacting operation, the outside surfaces of the green compact are not“moistened” by the auxiliary compacting agent, so as to prevent looseparticles from adhering.

The mixing with the metal powder does not result in disadvantages, evenat low molecular weights. The mixing operation can be influenced withinspecific limits through the selection of the auxiliary compacting agentand/or a mixture of auxiliary compacting agents with correspondingmolecular weight. Surprisingly, it has turned out that on the one handpolyethylene oxide can be mixed uniformly with metal powders, even atvery low molecular weights and small weight shares while, on the otherhand, it is possible to achieve a good “flowing” of the powder mixtureduring the mold filling.

The auxiliary compacting agent can be mixed “cold” into the metalpowder, meaning at room temperature. However, a warm mixing of theauxiliary compacting agent with the metal powder is particularly useful,e.g. in a heated drum mixer with subsequent cooling and simultaneousagitation. In that case, the temperature of the mixer is initiallyadjusted to be somewhat higher than the softening temperaturepredetermined for the compacting operation. It makes sense if the mixingtemperature is 50-100° C., preferably 85° C. Following the cooling down,a pourable powder mixture is then available, which ensures an easyhandling during the filling of the mold.

With a liquid consistency of the auxiliary compacting agent, anadditional reduction in the viscosity is possible by adding a solvent,so that the powder particles can be provided with an even thinnercoating of the auxiliary compacting agent, in a process that iscomparable to the spray drying process. Suitable solvents include, inparticular, alcohols such as ethanol, isopropanol, or benzyl alcohol,which evaporate quickly after the spraying, so that the resulting powdermixed with auxiliary compacting agent is “dry” and the requiredpourability or flowability for filling into the mold is maintained.

One advantageous embodiment of the invention provides that the mixturecontains a share of up to 5 weight % of the auxiliary compacting agent,relative to the share of metal powder. In that case, advantageous use ismade of the fact that the density of the auxiliary compacting agentaccording to the invention is higher than the density of traditionalauxiliary compacting agents. Thus, given the same weight share, thespace factor for the auxiliary compacting agent is adjusted lower andthe space factor for the compacted metal powder is consequently adjustedhigher. It makes sense to use an auxiliary compacting agent share of nomore than 1 weight %, relative to the metal powder.

The auxiliary compacting agent in the form of polyalkylene glycol,especially in the form of polyethylene glycol, is selected such that ithas a softening point between 40° and 80° C. The use of polyethyleneglycol products with molecular weights between 100 g/mol and 6500 g/mol,preferably 3000 to 6000 g/mol, has proven to be advantageous. It makessense in this case to use mixtures of polyethylene glycol with differentmolecular weights which, however, in the mixture should approximatelycorrespond to the total molecular weight.

The hydroxyl number for the auxiliary compacting agent can range between500 and 700, while the density can range between 0.9 and 1.25 g/cm³.

By mixing polyethylene glycols with various molecular weights, it ispossible to purposely arrive at an auxiliary compacting agent, which canbe adapted exactly to the compacting process used with respect to mixingqualities, softening point and lubricating properties.

The herein-suggested auxiliary compacting agent can be characterizedwith the following total formula:

H—[—O—CH ₂—CH₂ 13 ]_(n)—OH

The increases in the compacting densities that can be achieved with theherein specified auxiliary compacting agent do not primarily result froma temperature-dependent change in the physical properties of the metalpowder, as for the process described in the EP-A-O 375 627. Theseincreases are essentially due to an improvement in the lubricatingbehavior of the powder to be compacted, particularly between the moldcavity wall and the powder filling, given a respective temperaturecontrol at the compacting tools. Another advantage of the auxiliarycompacting agent suggested herein is that it can be eliminated easierthermally prior to the sintering, e.g. through diffusion processes, theescape via capillary forces, sublimation, evaporation or the like. Inthis connection, the auxiliary compacting agent according to theinvention also distinguishes itself by an environmentally acceptabledisposal option since it can be separated into water vapor and carbondioxide in a pyrolysis.

Surprisingly, it has turned out that an auxiliary compacting agentconsisting of a mixture of a traditional amide wax, present as a hardand extremely brittle powder, with a polyethylene glycol having amolecular weight of more than 7000 g/mol also leads to excellentcompacting results and an easy ejectability of the green compact fromthe mold. A “wetting” of the outside surfaces is avoided with certaintyin that case. The share of polyethylene glycol in the auxiliarycompacting agent mixture in this case can be considerably below 40%.Ethylene pis-stearoylamide can be used here as amide wax.

Metal stearates, particularly lithium stearate or zinc stearate as wellas paraffin products, waxes, natural or synthetic fat derivatives wereused until now as lubricants to reduce the friction between mold cavitywall and powder particles on the one hand and between powder particleson the other hand. For newer developments, multi-component,high-temperature resistant (meaning in this case approximately 130° C.)lubricants are used, which thus cause a reduction of the yield strengthof the metal to be compacted and consequently lead to higher compactingdensities, as previously described in the EP-A-O375 627. The followingdiagram shows a comparison of the moldability according to variousprocesses, conventional compacting at room temperature, so-called warmcompacting as described in the EP-A-O 375 627, and the process accordingto the invention.

One experiment uses a water-atomized iron powder containing 2% copperand 0.6% carbon, respectively in the form of a powder. The curvesschematically illustrate the dependence of the density on the compactingpressure.

The curve 1 as reference curve shows the result when using acold-compacting process with a traditional lubricant in the form of anamide wax or a microwax, e.g. ethylene pis-stearoylamide.

The curve 2 shows the result when using a warm-compacting processaccording to prior art. A clear improvement is already noticeable inthis case. However, the previously described disadvantages must be takeninto account here.

The curve 3 finally shows the result if the process according to theinvention is used, which leads to an even clearer increase in the finaldensity.

The following tables list green densities and green strengths that canbe achieved in dependence on the compacting pressure by contrasting theresults, obtained when subjecting a metal powder with auxiliarycompacting agent to different compacting pressures during differentmixing processes.

TABLE 1 Green compact Compacting Density strength in the pressure MixingCompacting in 3-point bending mpa operation operation g/cm³ test N/mm²400 cold cold 6.68 10.50 600 cold cold 7.07 13.40 800 cold cold 7.1416.80

TABLE 2 Green compact Compacting Density strength in the pressure MixingCompacting in 3-point bending mpa operation operation g/cm³ test N/mm²400 warm cold 6.80 14.30 600 warm cold 7.22 20.80 800 warm cold 7.3522.20

TABLE 3 Green compact Compacting Density strength in the pressure MixingCompacting in 3-point bending mpa operation operation g/cm³ test N/mm²400 cold warm 6.85 23.10 600 cold warm 7.24 34.10 800 cold warm 7.3335.80

TABLE 4 Green compact Compacting Density strength in the pressure MixingCompacting in 3-point bending mpa operation operation g/cm³ test N/mm²400 warm warm 6.88 25.60 600 warm warm 7.28 37.40 800 warm warm 7.3738.30

TABLE 5 Green compact Compacting Density strength in the pressure MixingCompacting in 3-point bending mpa operation operation g/cm³ test N/mm²400 cold cold 6.75 5.40 600 cold cold 7.07 6.70 800 cold cold 7.12 6.80

For the above mentioned metal powder, Table 1 shows a content of 0.6weight % of polyethylene glycol with a mol weight in the range ofapproximately 6000 g/mol, which is mixed and also compacted cold,meaning at room temperature. The table shows a nearly proportionalincrease of the green density and the green strength to the compactingpressure.

In Table 2, the result for a starting material with the same compositionis shown. However, the material was warm-mixed and cold compacted. Inaddition to an increase in the green density, a clear increase in thegreen strength is shown here as compared to the values for the coldcompacting of a cold-mixed powder. Using a mixing temperature in therange of the upper limit of the softening temperature for the compactingagent, or slightly above it, clearly results in a better distribution inthe mold cavity for the powder and thus a thinner “lubricant film,”which favors the sliding movements of the powder particles and thus alsothe “contact density” of the metal particles and the “interlocking” madepossible by this.

Table 3 shows the values for a cold-mixed powder that is warm-compacted.The achievable values for the green density correspond to theaforementioned values, while the green strength shows a clear increase,which demonstrates the interconnection between the type of polyethyleneglycol with low molecular weight that was used and the temperaturecontrol during the compacting.

Table 4, on the other hand, shows a further increase in the greendensity for a warm-mixed powder that is warm-compacted, wherein nearlythe maximum possible density near the density of solid iron is reachedfor a compacting pressure of 800 mpa. Particularly noticeable in thiscase, however, is the further increase in the green strength. The greenstrength was determined with a so-called 3-point bending test. Theindicated values respectively designate the specific applied load on thesurface, for which a break occurs in the green compact.

The improvement in the green density that is displayed in the precedingtables, particularly also the green strength, is probably due to the useof a polyethylene glycol with a molecular weight of less than 7000g/mol. Critical in this case is the increase in the green strengthoccurring during the warm-mixing, which is probably due to the fact thatduring the warm-mixing, the iron-powder particles, the copper particlesand the carbon particles are coated with an extremely thin coating ofthe auxiliary compacting agent. This is obvious from the fact that witha warm-mixed powder, the aforementioned composition of the carbon powderto be mixed in does not create any dust and, as compared to thecold-mixed powder, does not stick to the finger during a “finger test.”A test of the distribution of the alloy powder shares copper and carbonshowed a homogeneity, which corresponds to the homogeneity of adiffusion-alloyed metal power. A metal powder is shown herein, for whichinitially the iron powder and the powdered alloy components are mixedand the mixture is then pre-treated thermally to allow the alloy powderto bind to the iron powder, so that a separation is avoided. Theauxiliary compacting agent is only mixed in after that, during anadditional operational step.

The experiments demonstrate that the energy-intensive thermalpre-treatment of the powder mixture can be omitted with the processaccording to the invention, simply because the powdery alloy componentsare bonded inseparably and with good homogeneity to the iron particleswith the aid of the auxiliary compacting agents, especially during thewarm-mixing process. This also clearly demonstrates the advantage of theinvention.

The increase in the green strength is probably due to the improved flowbehavior under pressure and temperature of the auxiliary compactingagent with relatively low molecular weight in the metal powder moldcavity. This is due to the fact that a much higher frequency of directcontact between metallic surfaces of the individual metal particles onthe one hand occurs because of the extremely homogeneous mixture ofauxiliary compacting agent and metal powder and, on the other hand,because of the thin “lubricant agent film” that forms during the mixingand which is further reduced during the warm-pressing, thus making itpossible to achieve the initially described plastic deformation andinterlocking of the metal powder particles.

In contrast to Table 2, somewhat higher values surprisingly resulted foran auxiliary compacting agent mixture of amide wax having a share ofapproximately 40% of a polyethylene glycol with a molecular weight ofmore than 6000 g/mol, which was warm-mixed into the metal powder thatwas subsequently warm-compacted.

Table 5 shows as reference the values for the metal powder, into whichan amide wax is cold-mixed and which is cold-compacted.

What is claimed is:
 1. A process for producing sinterable, metallicshaped parts from a metal powder mixed with an auxiliary compactingagent, which contains at least in part components from the family ofpolyethylene glycols having a molecular weight of between 100 and 6,500g/mol and which is filled into a compacting mold and, after beingcompacted under pressure, is ejected as a compacted shaped part from themold.
 2. A process according to claim 1, characterized in that theauxiliary compacting agent share present in the mixture is up to 5weight %, preferably less than 1 weight %, relative to the metal powdershare.
 3. A process according to claim 1, characterized in that theauxiliary compacting agent has a softening point between 40° C. and 80°C.
 4. A process according to claim 1, characterized in that theauxiliary compacting agent has a molecular weight of between 3000 and6000 g/mol.
 5. A process according to claim 1, characterized in that theauxiliary compacting agent comprises less than 40% polyethylene glycol.6. A process according to claim 1, characterized in that the auxiliarycompacting agent has a hydroxyl number of 5 to
 700. 7. A processaccording to claim 1, characterized in that the auxiliary compactingagent has a density of 0.9 to 1.25 g/cm³.
 8. A process according toclaim 1, characterized in that the metal powder mixed with the auxiliarycompacting agent is filled into the compacting mold at a temperaturebelow the softening point of the auxiliary compacting agent used, sothat the auxiliary compacting agent is softened by energy introducedinto the compacting mold during at least one step of the process,wherein the at least one step includes the compacting (warm-pressing).9. A process according to claim 1, characterized in that the metalpowder mixed with the auxiliary compacting agent is filled into thecompacting mold at a temperature below the softening point for theauxiliary compacting agent used and is compacted without supplyingadditional energy during the compacting (cold-pressing).
 10. A processaccording to claim 1, characterized in that the auxiliary compactingagent is mixed into the metal powder at a temperature which is at leastin the range of the softening point for the auxiliary compacting agent(warm-mixing).